One of the incipient problems that has been plaguing the jet engine is the coke buildup particularly in the internal areas of the fuel nozzles. For this reason, the time interval between overhaul or repair or removal of these nozzles is not as long as it might be. Obviously, from a maintenance standpoint, this is not only a costly problem but a complex one since in many engines, a good part of the engine has to be torn down to get at these nozzles. Furthermore, coke buildup changes the nozzle spray characteristics affecting the efficiency of its operation, impairing the engine's overall operational efficiency and life.
Although the problem has persisted for a considerable time and many attempts to solve it have been made, none heretofore have met with any success. Typically, means have been provided to wash away external carbon deposits, as by blowing air over the surface where the deposition is apt to occur. Obviously, this solution anticipates the deposition of the carbon first and the blowing of air to remove the same. An example where this solution is described is in U.S. Pat. No. 3,788,067 granted to D. R. Carlisle and J. J. Nichols on Jan. 29, 1974. These solutions are generally applied where fuel tends to accumulate on the nozzles' surfaces during engine operation and after the engine is shut down. Upon operation and restarting, air is blown over those surfaces to remove any fuel residue.
We have found that we can obviate the problem in dual orifice nozzles, that is, in nozzles where there are primary and secondary fuel passages, where the primary or pilot nozzle is continuously operative and the secondary or main nozzle is only operative on the higher thrust levels of engine operation. For example, our invention has been particularly efficacious in fuel nozzles for such engines like the JT-8D and JT-9D manufactured by the Pratt and Whitney Aircraft Group of United Technologies Corporation. This invention contemplates pressurizing or increasing the pressure within the secondary fuel passage when only the primary fuel passage is operative. In this mode, flow of fuel from the primary passage and the surrounding airflow behaved as a jet pump creating a negative pressure in the secondary passage inducing fuel flow egressing from the primary nozzle to migrate therein and hence manifesting the buildup of coke.
The comprehension of this problem has been evasive to many people who attempted to solve it. Since the problem was never fully understood, its solution was not readily apparent. Thus, we have found that by the proper circuiting of airflow during the low thrust regimes, the air can be directed to build up the pressure in the secondary passage, eliminate the negative pressure heretofore created therein and prevent fuel from digressing therein.